1. Field of the Invention
The present invention relates to implants for the human body and, more particularly, but not by way of limitation, to methods and an apparatus for an implant having multiple transition temperatures.
2. Description of the Related Art
Shape memory alloys such as nitinol have been well known since their development in 1965 by Buehler and Wiley (U.S. Pat. No. 3,174,851). Other metals, such as AuCd, FePt.sub.3, beta Brass, and InTI, exhibit shape memory behavior. These materials have the property of changing shape in response to a change in material temperature. This shape change potential is imparted into the memory metal device through a series of heat treatments.
The transition temperature range is imparted to the material through varying mixtures of intermetallic compounds such as nickel-titanium and heat treatment. The heat treatment methods for the material generally consist of a high temperature setting of the desired final shape of a device followed by a low temperature straining of the device to a second shape. Then, when the device is in the second shape and brought to the transition temperature, the device returns to the preprogrammed final shape. The shape change occurs due to the transition of the material from a martensitic to austenitic phase microstructure. These heat-initiated changes cause gross changes in the shape of the implant formed from the memory metal.
Shape memory alloys have been used for a wide range of industrial and medical applications. Medical applications include but are not limited to: catheter, intrauterine contraceptive device, gastrointestinal compression clip, blood vessel filter, coronary artery stent, skin staple, bone staple, and bone plate. In medical applications, shape memory alloys are generally designed so that they change shape once when heated to and beyond a specific temperature. The implants and devices are designed as a whole to transition once from martensite to partial or full austenite. For example, Fox (U.S. Pat. No. 7,240,677) describes a method for force, displacement, and rate control of shaped memory metal implants. Nevertheless, the implants and techniques in this patent do not describe multiple transition temperatures in the same device.
However, in many instances, it may be desirable for an implant or device to have either multiple transition temperatures, or multiple elements that transition at different temperatures. The existence of multiple transition temperatures would allow, for example, complex devices that can be heated first to one shape, and then heated further to additional shapes. Medical devices in orthopedics could be designed so that they undergo sequential shape changes for complex treatment of bones. Devices could also be designed such that part of the device is intentionally left in martensite. A device that has a portion that is always martensitic would be helpful in creating implants that can be deformed to conform to the curvature of bone. Other devices could be designed such that there is a shape changing portion that is martensitic at room temperature, and a second portion that does not change shape when heat is applied because it is always austenitic at normal temperatures.
Accordingly, a shape memory implant or device that features multiple transition temperatures or multiple elements with different transition temperatures would be beneficial to surgeons, as well as persons requiring bone surgeries, because the shape changing features of the device can be more complex and sequentially applied.